Semiconductor substrate having stress-absorbing surface layer

ABSTRACT

An assembly ( 101 ) comprising a semiconductor device ( 110 ) with solderable bumps ( 112 ); a substrate ( 120 ) with a layer ( 130 ) of a first insulating compound and an underlying metal layer ( 140 ) patterned in contact pads ( 141 ) and connecting traces ( 142 ), the insulating layer having openings ( 132 ) to expose the surface ( 142   a ) and sidewalls ( 142   b ) of underlying traces; the device bumps soldered onto the contact pads, establishing a gap ( 150 ) between device and top insulating layer; and a second insulating compound ( 160 ) cohesively filling the gap and the second openings, thereby touching the underlying traces, the second insulating compound having a higher glass transition temperature, a higher modulus, and a lower coefficient of thermal expansion than the first insulating compound.

CROSS REFERENCE SECTION

This application claims the benefit of Provisional Application No.61/847,631, filed Jul. 18, 2013, and the entirety of which is herebyincorporated by reference.

FIELD

Embodiments of the invention are related in general to the field ofsemiconductor devices and processes, and more specifically to thestructure and fabrication method of semiconductor packages with hybridsolder masks capable of arresting nascent cracks in laminate traces.

DESCRIPTION OF RELATED ART

In semiconductor devices referred to as flip chip devices, terminals ofsemiconductor chips are attached to substrate pads with arrays of solderballs, or bumps, rather than the traditional metal wires. The number ofthe resulting input/output terminals may vary from only a handful—forwhich conventional metallic leadframes are preferred as substrates—toseveral hundred, for which laminated substrates are preferred. Thelaminated substrates are made of a vertical hierarchy of insulatinglayers alternating layers of patterned metal traces, which caninterconnect from layer to layer with conductive vias through theinsulators.

Similarly, in semiconductor devices referred to as ball grid arraydevices, terminals of packaged semiconductor chips are attached tosubstrate pads with arrays of solder balls, or bumps, rather than thetraditional metal pins. It has been known since the introduction ofthese assembly techniques that the solder connections may come undersevere thermomechanical stress due to the large differences ofcoefficients of thermal expansion (CTE) between semiconductor materials,such as silicon, and plastic materials, such as packages and substrates,and metals, such as copper.

It is common practice to test the reliability of the solder connectionsby subjecting the assembled semiconductor devices to temperature cycles,where the assembled packages are subjected to rapid temperature swingsbetween −55° C. and +125° C. It has been shown that these temperatureswings subject the assemblies to both compressive and tensile stressesand may lead to metal fatigue and eventual cracks at the joints.

SUMMARY

When applicants analyzed failures occurring in reliability temperaturecycling tests of semiconductor devices attached to laminated substratesby solder bumps, they found that the patterned traces of the top metallayer of the substrate have a propensity to crack and thus fail due toelectrical open. The analysis of failures revealed that stresses due topackage warpage and high stress concentrations at the package cornersare transferred to the top substrate layer and then transmitted throughthe top insulating layer to the underlying conductive traces. Afterrepeated cycles, the thin metallization layer of the traces succumbs tothe stresses, then develops microcracks, and finally cracks open.

Applicants solved the cracking problem when they discovered amethodology to reduce the stresses by about 25%, when the substrate topinsulation layer of a first insulating compound is transformed into acomposite aggregate, in which areas of the first insulating compoundalternate with areas of a second insulating compound and the secondcompound has a higher glass transition temperature T_(g), a highermodulus, and a lower coefficient of thermal expansion (CTE) than thefirst compound.

As applicants found, the composite aggregate can be achieved by creatingopenings in the top insulating layer of the first insulating compound sothat the surface of the underlying conductive traces is exposed, oralternatively, the surface and the sidewalls of the underlying tracesare exposed. The openings are then filled with a second insulatingcompound touching the traces, wherein the second insulating compound hasa higher glass transition temperature T_(g), a higher modulus, and alower coefficient of thermal expansion (CTE) than the first compound.

Alternatively, the openings in the first insulating compound are filled,together with the gap between the device and the substrate created bythe solder ball assembly, with the underfill compound, which has ahigher T_(g), higher modulus, and lower CTE than the first insulatingcompound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross section of an embodiment where the underfillcompound touches the surface of the routing traces.

FIG. 1B depicts a cross section of another embodiment where theunderfill compound touches the surface and the sidewalls of the routingtraces.

FIG. 2 a cross section of yet another embodiment where astress-absorbing compound fills the recess of the solder mask to therouting traces

FIG. 3 shows a Table with examples of typical materials properties ofinsulating compounds used in semiconductor device assembly.

FIG. 4 is a plot of tensile stress at packaged semiconductor chips as afunction of distance from the chip corner, showing the effect ofstress-absorbing compounds.

FIG. 5A illustrates a cross section of a substrate for semiconductordevices having a surface layer of a first insulating compound withopenings to expose the surface of underlying conductive traces.

FIG. 5B shows a cross section of a substrate for semiconductor deviceshaving a surface layer of a first insulating compound with openings toexpose the surface and the sidewalls of underlying conductive traces.

FIG. 5C illustrates a cross section of a substrate for semiconductordevices having a surface layer of a first insulating compound withopenings to expose the surface of underlying conductive traces, theopenings filled with a stress-absorbing second insulating compound.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A shows a semiconductor assembly 100 as an exemplary embodiment ofthe invention; another embodiments 101 is displayed in FIG. 1B, and yetanother embodiment 200 in FIG. 2. Assembly 100 includes a semiconductordevice 110 attached to a substrate 120. Devices 110 may be packagedsemiconductor products, or they may be semiconductor chips having anactive surface such as an integrated circuit. Devices 110 include aplurality of terminals 111, which have a metallurgy suitable for solderattachment. Terminals 111 include metal bumps 112, which are alsosolderable. Exemplary FIGS. 1A and 1B depict the metal bumps 112 assolder balls; in other devices, the metal bumps may include copperpillars with solder caps, or, in still other examples, squashed copperor gold balls as created in wire bonding processes, also in combinationwith solder caps.

Substrate 120 is flat and rigid so that substrate warpage during usageand testing of the assembly is minimized. Substrate 120 may comprise acomposite board with a plurality of laminated alternatingly conductiveand insulating films. The conductive films are patterned in horizontalinterconnecting traces, and the insulating films have conductivevertical vias. Alternatively, substrate 120 may be a carrier such as aninsulating stiff board.

In the exemplary embodiment, substrate 120 includes a top layer 130 of afirst insulating compound. A preferred thickness range for layer 130 isabout 14 to 30 μm. As an example, first insulating compound is apolymeric and filler-filled material commonly known as solder mask orsolder resist. The Table in FIG. 3 compiles a few material properties ofan exemplary solder resist compound as follows (approximate numbers): Aglass transition temperature (T_(g)) of 102° C., a modulus of 2.7 GPa, acoefficient of thermal expansion (CTE) of 55 ppm/° C. before reachingT_(g), and a CTE of 140 ppm/° C. after reaching T_(g), a tensilestrength of 48 MPa, and a capability for elongation of 3.6%. Thesematerial characteristics are interrelated; for instance, a higher T_(g)and a higher modulus are related to a lower CTE. A guiding fact in theselection of proper materials is the discrepancy of about a factor of 10or more between the low CTE of silicon and the higher CTE of plasticsand metals. As for the elongation, it is a parameter expressing how muchan outside force, applied along the length of a lead, can stretch thelead in the direction of the length, while the dimension of the width isonly slightly reduced so that the new shape appears elongated. Forelongations small compared to the length, and up to a limit called theelastic limit given by the material characteristics, the amount ofelongation is linearly proportional to the force. Beyond that elasticlimit, the lead suffers irreversible changes and damage to its innerstrength and will eventually break.

Covering a wider range of materials, the first insulating compoundpreferably is a rigid polymer having a modulus typically in the range of2 to 6 GPa at room temperature and a CTE in the range of 40 to 70 ppm/°C. below the glass transition temperature of the material, and thesecond insulating compound is an epoxy-based polymer containing aninorganic filler material and having a modulus typically in the range of6 to 11 GPa at room temperature and a CTE in the range of 20 to 40 ppm/°C. below the glass transition temperature.

In FIGS. 1A and 1B, right underneath and touching insulating top layer130 is metal layer 140. Relative to insulating substrate 120, metallayer 140 represents the topmost conductive layer. A preferred thicknessrange for layer 140 is between about 10 and 20 μm. Preferred metals forlayer 140 are copper and copper alloys; alternatively, layer 140 may bealuminum. As FIGS. 1A and 1B show, layer 140 is patterned in contactpads 141 and connecting traces 142.

As FIGS. 1A and 1B illustrate, insulating layer 130 has first openings131 to expose underlying contact pads 141, and second openings 132 toexpose underlying connecting traces 142. In the embodiment of FIG. 1A,second openings 132 expose the surface 142 a of the underlyingconnecting traces 142. In the embodiment of FIG. 1B, second openings 132expose the surface 142 a and the sidewalls 142 b of the underlyingconnecting traces 142.

It should be pointed out that this methodology of forming astress-absorbing hybrid insulating layer can be applied repeatedly.Numerous consecutive and contiguous zones can be created including afirst and a second insulating material, wherein one material has ahigher T_(g), a higher modulus, and a lower CTE than the other material.

In contrast to FIGS. 1A and 1B, the second openings 132 do not exist inpresent technology; rather, solder mask layer 130 has a flat surfacebetween the first openings so that traces 142 are covered with firstinsulating compound.

As shown in FIGS. 1A and 1B, device 110 is assembled on the substrate bysoldering the device bumps 112 through first openings 131 onto thecontact pads 141. In this attachment process, a gap 150 is createdbetween the underside of device 110 and the top surface of the topinsulating layer 130. For the assemblies shown in the figures, it ispreferred that gap 150 is about 50 to 100 μm high; in other devices, gap150 may be shorter or higher. The actual width of gap 150 depends on thesurface contours of chip 110 and substrate 120.

In a process based on capillary action and frequently calledunderfilling process, gap 150 is cohesively filled with a secondinsulating compound 160, which touches and preferably adheres to themetallic, semiconductor, and insulating surfaces facing gap 150.Consequently, compound 160 touches and preferably adheres to thesurfaces 142 a of the connecting traces 142 in FIG. 1A, or touches andpreferably adheres to the surfaces 142 a and sidewalls 142 b of theconnecting traces 142 in FIG. 1B.

The second insulating compound 160 has a higher glass transitiontemperature (T_(g)), a higher modulus, and a lower coefficient ofthermal expansion (CTE) than the first insulating compound employed forlayer 130. Table 1 in FIG. 3 compiles a few material properties of anexemplary second insulating compound as follows (approximate numbers): Aglass transition temperature (T_(g)) of 120° C., a modulus of 10.7 GPa,a coefficient of thermal expansion (CTE) of 29 ppm/° C. before reachingT_(g), and a CTE of 100 ppm/° C. after reaching T_(g), a tensilestrength of about 102 MPa, and a capability for elongation of 1.9%. Aswith the first insulating compound, these material characteristics areinterrelated; for instance, a higher T_(g) and a higher modulus arerelated to a lower CTE. As a comparison with the materialscharacteristics of the first insulating compound shows (see the Table ofFIG. 3), the CTE before T_(g) of the second compound is less than afactor of 10 higher than the CTE of silicon. Consequently, the secondinsulating compound expands and contracts considerably less and thuscreates much less thermomechnical stress than the first insulatingcompound, and is furthermore able to absorb thermomechanical stress tosuch extent that cracking of solder mask and of metallic parts (such asinterconnecting traces, solder joints, adhering parts) can be prevented.

FIG. 4 displays modeling data confirming the stress reduction effect ofthe second insulating compound and the significant enhancement of thiseffect by covering traces 142 with second insulating compound so thattraces 142 are confronted with much reduced stress levels. As anindicator of the stress intensity, the double-logarithmic plot of FIG. 4shows the tensile stress (in MPa) as a function of the distance (in mm)from the corner of chip 110. The tensile stress is determined at thecopper trace 142. The modeling data interpolation is extrapolated to thechip corner, where the stress has a maximum value. The stress data iscompared between device assemblies where the substrate does include(invention) solder mask openings 132 filled with the second insulatingcompound to the traces 142 (FIGS. 1A and 1B), and where the substratehas just solder mask instead of openings 132.

As the modeling data show, the stress intensity can be interpolated tofollow straight lines of different slopes. When extrapolated to the chipcorner, the data indicate that the second insulating compound, fillingin opening 142, reduces the tensile stress at the copper traces by about25%. This reduction is sufficient to prevent the appearance ofmicrocracks in traces 142 and thus to prevent openings in metallictraces.

FIG. 2 depicts another embodiment 200 comprising a semiconductorassembly. A semiconductor device 210 has terminals 211 with solderablemetal bumps 212. A flat substrate 220 includes a top layer 230 of afirst insulating compound and an underlying metal layer 240 patterned incontact pads 241 and connecting traces 242. The insulating layer 230includes first openings 231 to expose underlying contact pads 241 andsecond openings 232 to expose underlying connecting traces 242.

As FIG. 2 illustrates, second openings 232 are filled with a thirdinsulating compound 270 so that the surface 270 a of the fill material270 is coplanar with the surface 230 a of top layer 230 of the firstinsulating compound. The openings 232 in the solder mask 230 are filledin with a third insulating compound 270, before substrate 220 is usedfor the assembly of device 200 and before underfill compound 260 isapplied. The third insulating compound 270 has a higher glass transitiontemperature (T_(g)), a higher modulus, and a lower coefficient ofthermal expansion (CTE) than the first insulating compound 230.Preferably, third insulating compound 270 has a higher glass transitiontemperature (T_(g)), a higher modulus, and a lower coefficient ofthermal expansion (CTE) than the second insulating compound 260 used forthe underfill process described below.

Device 210 is assembled on substrate 220 by a solder reflow process,wherein the device bumps 212 are soldered through the first openings 231onto the contact pads 242. By this reflow attachment process, a gap 250is established between the device 210 and the top insulating layer 230.

In the so-called underfill process based on capillary force, a secondinsulating compound 260 fills gap 250, the second insulating compoundhaving a higher glass transition temperature (T_(g)), a higher modulus,and a lower coefficient of thermal expansion (CTE) than the firstinsulating compound 230.

Another embodiment of the invention is a substrate for use insemiconductor devices. Exemplary substrates are depicted in FIGS. 5A,5B, and 5C. Substrate 520 is flat and rigid so that substrate warpageduring usage and testing of the assembly is minimized. Substrate 520 maycomprise a composite body with a plurality of laminated areas ofalternatingly conductive and insulating films. The conductive films arepatterned in horizontal interconnecting traces, and the insulating filmshave conductive vertical vias. Alternatively, substrate 520 may be acarrier such as an insulating stiff board.

In either case, flat body 520 has a top layer 530 of a first insulatingcompound with a surface 530 a, and an underlying metal layer 540patterned in contact pads 541 and connecting traces 542. The firstinsulating compound may be a polymeric compound filled with an inorganicfiller to provide mechanical strength and lower CTE; a preferredcompound is the so-called solder mask or solder resist. Preferred metalsof layer 540 are copper or a copper alloy due to their high electricalconductivity. Alternatively, layer 540 may include silver and aluminum.

Top insulating layer 530 has first openings 531 to expose underlyingcontact pads 541 and second openings 532 to expose underlying connectingtraces 542. In FIG. 5A, the underlying traces 542 are exposed so thatthe surface 542 a of the traces is exposed. In FIG. 5B, the underlyingtraces 542 are exposed so that the surface 542 a and the sidewalls 542 bof the traces are exposed.

In FIG. 5C, the second openings 532 are filled with a third insulatingcompound with a higher glass transition temperature (T_(g)), a highermodulus, and a lower coefficient of thermal expansion (CTE) than thefirst insulating compound 530. As FIG. 5C shows, third insulatingcompound 570 is in touch with the underlying connecting traces 542.Furthermore, the surface 570 a of the filled second openings arecoplanar with the surface 530 a of the first insulating layer.

The substrate of FIG. 5C has a hybrid stress-absorbing surface layer andacts as a substrate with a hybrid solder mask capable of arrestingnascent cracks in laminate traces and thus preventing failures inoperations and tests generating thermomechnaical stresses.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the invention is applicable for substratesfor any semiconductor flip-chip or ball grid array device, includingsilicon germanium and III-V compound semiconductors.

As another example, the invention is applicable for any assembly usingmetals (such as copper pillars and gold bumps) for interconnecting andspacing devices on substrates, and where CTE differences causethermomechanical stresses. As another example, the invention applieswhere hybrid insulating layers composed areas of different T_(g),modulus, and CTE may be used as stress-absorbing layers incorporatedeven inside of substrates.

It is therefore intended that the appended claims encompass any suchmodifications or embodiments.

We claim:
 1. A semiconductor assembly comprising: a semiconductor devicehaving terminals with solderable metal bumps; a flat substrate having atop layer of a first insulating compound and an underlying metal layerpatterned in contact pads and connecting traces, the insulating layerhaving first openings to expose underlying contact pads and secondopenings to expose underlying connecting traces; the device assembled onthe substrate wherein the device bumps are soldered through the firstopenings onto the contact pads, thereby establishing a gap between thedevice and the top insulating layer; and a second insulating compoundcohesively filling the gap and the second openings, thereby touching theunderlying connecting traces, the second insulating compound having ahigher glass transition temperature (T_(g)), a higher modulus, and alower coefficient of thermal expansion (CTE) than the first insulatingcompound.
 2. The assembly of claim 1 wherein the first insulatingcompound is a rigid polymer having a modulus typically in the range of 2to 6 GPa at room temperature and a CTE in the range of 40 to 70 ppm/° C.below the glass transition temperature of the material, and the secondinsulating compound is an epoxy-based polymer containing an inorganicfiller material and having a modulus typically in the range of 6 to 11GPa at room temperature and a CTE in the range of 20 to 40 ppm/° C.below the glass transition temperature.
 3. The assembly of claim 1wherein the second openings expose the surface of the underlyingconnecting traces.
 4. The assembly of claim 3 wherein the secondopenings include more than one opening for each connecting trace.
 5. Theassembly of claim 1 wherein the second openings expose the surface andthe sidewalls of the underlying connecting traces.
 6. The assembly ofclaim 1 wherein the semiconductor device is a semiconductor chip.
 7. Asemiconductor assembly comprising: a semiconductor device havingterminals with solderable metal bumps; a flat substrate having a toplayer of a first insulating compound and an underlying metal layerpatterned in contact pads and connecting traces, the insulating layerhaving a surface with first openings to expose underlying contact padsand second openings to expose underlying connecting traces; a thirdinsulating compound filling the second openings, the third insulatingcompound having a higher glass transition temperature (T_(g)), a highermodulus, and a lower coefficient of thermal expansion (CTE) than thefirst insulating compound, the surface of the filled opening coplanarwith the surface of the first insulating layer; the device assembled onthe substrate wherein the device bumps are soldered through the firstopenings onto the contact pads, thereby establishing a gap between thedevice and the top insulating layer; and a second insulating compoundcohesively filling the gap except the second openings, the secondinsulating compound has a higher T_(g), a higher modulus, and a lowerCTE than the first insulating compound.
 8. The assembly of claim 7wherein the third insulating compound has a higher T_(g), a highermodulus, and a lower CTE than the second compound.
 9. A substrate foruse in semiconductor devices comprising: a flat body having a firstlayer of a first insulating compound, and an underlying metal layerpatterned in contact pads and connecting traces, the first insulatinglayer having first openings to expose underlying contact pads and secondopenings to expose underlying connecting traces.
 10. The substrate ofclaim 9 wherein the second openings expose the surface of the underlyingconnecting traces.
 11. The assembly of claim 9 wherein the secondopenings expose the surface and the sidewalls of the underlyingconnecting traces.
 12. The substrate of claim 9 wherein the secondopenings are filled with a third insulating compound having a higherglass transition temperature (T_(g)), a higher modulus, and a lowercoefficient of thermal expansion (CTE) than the first insulatingcompound, the third insulating compound in touch with the underlyingconnecting traces.
 13. The substrate of claim 12 wherein the surface ofthe filled second openings are coplanar with the surface of the firstinsulating layer.